Process for regenerating a nitric acid-hydrofluoric acid pickling solution

ABSTRACT

In pickling metal, such as iron, steel, special alloys, or the like, aqueous blends containing hydrofluoric acid and nitric acid are used as pickling solutions; resulting spent pickling solutions are evaporated under controlled conditions, such that the concentrate is supersaturated and contains a predetermined amount of iron; the concentrate is then partially cooled and transferred to a crystallization zone where, under controlled conditions, a crystalline precepitate is formed by cooling the concentrate in two phases; the resulting precipitate is then separated from its mother liquor, and the latter is recycled for use in pickling metals of the aforedescribed type.

The present application is a continuation-in-part of application Ser.No. 665,227, filed Mar. 9, 1976, now abandoned, by the present applicantand bearing the same title.

BACKGROUND

For economy and ecological reasons, numerous efforts have been madeheretofore to recover, from spent mixed acid (i.e. HF/HNO₃) solutions,valuable chemicals which can be used for pickling purposes, and to copewith serious ecologically undesirable waste sewage problems. Inherently,recycling of hydrofluoric acid and nitric acid would provide realeconomy if recovery thereof and recycling could be achievedeconomically.

Unfortunately, processes proposed and/or adopted in recent years areexpensive, cumbersome, involved, or all of these.

Typically, one process which has been recommended would use, e.g.,sodium fluoride to precipitate the double salt Na₂ FeF₅, separation ofthe latter and treating it with caustic soda in order to separate ferrichydroxide and recover recyclable sodium fluoride. The process is veryhighly expensive because of the presence of other metal fluorides andthe requirement that a partial stream of any recycled solution betreated with caustic soda.

In another proposed process a strongly basic ion exchange resin iscontacted by the spent pickling solution and the ion exchange resinwashed with water. As with a chromatographic column, the effluentexhibits two partly overlapping concentration peaks, the first of whichis due to the predominating quantity of salts and the second to thepresence of free acids. An optimal separation of fractions allegedlyenables a recovery of about 60% of free nitric acid present. Theacid/salt fraction must be treated with neutralizing agents. This methodlikewise does not really solve the problem because the recovery rate isunsatisfactory.

In a third suggested method, the nitric and hydrofluoric acid residuescombined with metal ions are first released by the addition of sulfuricacid in a corresponding quantity, and free acids plus dissolvedmolybdenum are extracted with a solution of tributyl phosphate inkerosene. The acids are eluted with water from the organic solvent phaseand the resulting aqueous solution is treated with activated carbon,after which it is returned to the cycle as regenerated solution. Beforethe organic phase can again take up acids, molybdenum fluoride must beremoved therefrom by treatment with a sodium hydroxide solution. Theresulting basic solution, together with an extraction residue whichcontains a major part of the heavy metals, is first neutralized withlime and then with a sodium hydroxide solution. It is apparent from thisshort description that detailed technological problems are involved, aswell as considerable expenditures for equipment, control mechanisms, andchemicals.

As is well known, spent pickling solutions contain various metal ions.In addition to ferric ion, the quantity of which predominates, thesolutions contain mainly chromium, nickel, and molybdenum; also presentmay be copper, vanadium, tungsten, cobalt, manganese, and many others,including titanium. Titanium, for example, can be treated, say, in theform of sheet titanium, with the same solutions used on high-gradesteels. For reasons of pickling technology, resulting spent solutionsmust be discarded when they contain only 40-50 g/l ferric ion, althoughthey may then still contain unused nitric acid in high concentrations,e.g., 150 g/l. This requirement creates great problems involvingneutralization and waste sewage disposal. Additionally, of course,operating costs, owing to poor utilization of chemicals, inter alia, arevery substantial.

As will be evident from the description, infra, of the presentinvention, a surprisingly straightforward and simple, and equallysurprisingly effective, process has been discovered. In other words,major problems in the nature of those inherent in the prior art areobviated. Furthermore, the novel process which will be describedhereinafter differs from this prior art in that extraneous acid,extraneous salt, extraneous solvent, or the like, are not added to spentpickling solutions; further, the solutions are not contacted with anionexchange resins, which resins are never perfectly resistant to nitricacid solutions.

INVENTION

The present invention relates to a process and apparatus for recoveringand recycling nitric acid and hydrofluoric acid from spent aqueoussolutions resulting from surface treatment (pickling) of iron,high-grade steel, special alloys, and the like. More particularly, themethod of the recovery which may be continuous or batch, involvessupersaturating, under controlled conditions, spent pickling solutionsby distillation; transferring the concentrate to a crystallization zone,and, again under carefully controlled conditions, crystallizing to forma precipitate of metal salts, e.g., FeF₃, CrF₃ ; separating the metalsalt precipitate from its mother liquor; and recycling the latter to apickling zone.

Even more particularly, the process of the present invention comprisestransferring from a pickling zone spent aqueous pickling solutioncontaining mixed acids having a temperature in the range of about 30° C.to about 80° C., generally about 40°-60° C., to an evaporation zone;heating said spent solution to boiling (generally about 105° C.), andthus distilling said spent solution in the evaporation zone, the boilingtemperature rising upon vaporization (generally to about 107.5° C.);removing the resulting supersaturated concentrate from the evaporationzone; cooling said concentrate to a temperature in the range of about50° C. to about 100° C., preferably about 70° C. to about 78° C.;introducing same into a crystallization zone, whereupon furtherreduction of temperature to about 30° C.-70° C., preferably about 40° C.to about 60° C., is effected; stirring the concentrate in thecrystallization zone at said temperature for a period of about 3 to 15hours, preferably about 4 - 12 hours, to effect the initialcrystallization; then, after said initial and rapid crystallizationduring which the major and first-phase crystallization takes place,decreasing the temperature of the concentrate to a temperature in therange of about 15° to about 30° C., preferably about 18° C. to about 25°C., to effect an increased second-phase crystallization, while likewisestirring, the second phase time period being about the same length asthat of the first phase; separating the resulting cake precipitate fromits mother liquor, such as by filtration; and recycling said motherliquor to the pickling (surface treatment) zone.

It has been found that crystallization takes place more rapidly in thefirst phase, i.e., at a temperature in the range of about 30° C. toabout 70° C. In fact, during first-phase crystallization as suggestedabove, the major part of the crystallization takes place. It has alsobeen discovered that it is highly advantageous to carry out thesecond-phase crystallization in the above-noted lower temperature rangeto effect more complete crystallization, i.e., an increase in crystalsdue to decreased solubility of crystallizable components (e.g. iron) atthese lower temperatures.

Supersaturation of the distillation residue (concentrate) created in thevaporization zone is critical. It is important to note here thatconventional evaporation/crystallization processes useful in otherfields of chemical technology are unsatisfactory to effect recovery ofvaluable chemicals from spent pickling solutions. In other words, simpleevaporation close to saturation, followed by cooling withcrystallization and supersaturation, is not effective.

As the spent pickling solution is evaporated, water which is onlyslightly contaminated is distilled off first. This water need not bereturned into the cycle. Only when a certain increase in concentrationhas been effected does the distillate contain hydrofluoric and nitricacids in such a concentration that return of the distillate to thesolution cycle constitutes an advantage, which advantage should beutilized.

The solutions to be processed vary somewhat in concentration. This ismainly due to the different contents of the various metals in, forexample, high-grade steels, special alloys, and semi-finished titaniumproducts, and also to the concentrations to which the acids have beenadjusted. It is sometimes endeavored to produce special technologicaleffects by an addition of some sulfuric and hydrochloric acids. Theprocess may also be applied to such solutions if nitric acid andhydrofluoric acid and dissolved iron are present in substantialquantities.

It has been found desirable in most cases to maintain the dissolved ironcontent in the distillation residue between 60 and 150 g/l, preferablybetween 80 and 120 g/l. Crystallization is sufficiently fast under thosecircumstances. It has been found that crystallization takes place morerapidly the higher the iron concentration in the concentrate.

The temperature of crystallization may be controlled by internal orexternal heating and cooling means, if ambient conditions are nothelpful. According to one embodiment, wherein cooling of theconcentrate, as indicated supra, is effected to about 50° C.-100° C.,before it enters the crystallization zone, temperature control in thecrystallization zone can be maintained by providing one or moreadditional successive crystallization zones. For example, using twocrystallization zones, the first zone is fed and kept filledcontinuously and temperature maintained during first-phasecrystallization by incoming hotter concentrate. Overflow from the firstcrystallization zone is directed to the second crystallization zone,thus effecting a drop in temperature but maintaining a relativelyconstant second phase temperature by virtue of the likewise incomingrelatively hotter concentrate.

More than two crystallization zones are preferred to avoid the situationwhere residence time in the first crystallization zone is inadequatewith respect to a portion of the concentrate which migrates too soon tothe overflow conduit and thus is cooled prematurely. This would notpermit sufficient crystallization to take place as to it.

If four or six successive crystallization zones are established, flowand overflow could be so controlled as to provide an average temperaturein the first two or three zones, respectively, of about 30° C. to 70° C.(first phase) and an average temperature in the last two or three zones,respectively, of about 15° C.-30° C. (second phase). In this wayresidence times in both phases and for the entire concentrate can besubstantially uniform.

As is evident, in the continuous system provision should be made toperiodically remove crystals from the mother liquor.

Because iron fluoride is the main constituent of the crystallineprecipitate, the concentration of iron in the concentrated solutionleaving the evaporator can be an important variable for the purpose ofcontrolling the extent of evaporation. It has been found, however, thatother variables which are more or less closely related to the ironconcentration, such as the fluorid ion concentration or density, andwhich can be more easily determined, may alternatively be used as acontrol indicator.

The surprising result reproduced by the process of the present inventionresides in that, contrary to previous experience, solutions can besupersaturated in continuous operation in an evaporator under certaincircumstances, and the concentration of the concentrate can be socontrolled that readily separable fluorides are precipitated therefrom.This technology requires at least initially a control of the residencetime of the solution in the evaporator. In batch operation, theevaporation is simply continued for a certain maximum time. Incontinuous evaporation, a check can be made by the addition of anon-reactive substance to the solution at the inlet of the evaporator.The residence time spectrum can then easily be established from the timefunction diagram of the concentrations of said substance in theevaporator effluent. Under conditions equal in other respects, theresidence time may be varied, e.g., in reciprocal dependence on thesolution rate. It has been found necessary to limit the residence timeof the solution in the evaporator to less than 60 minutes, preferablyless than 15 minutes. After this time all the non reactive substanceadded must have left the evaporator. It will be understood that incontinuous evaporation it is essential that the residence time spectrumof the solution in the evaporator be as sharp as possible. It has beenfound particularly desirable to conduct the solution through theevaporator in one or more flow channels and to heat the solutionpreferably by direct electric resistance heating with alternatingcurrent.

It is not necessary to use very high frequencies for avoiding anelectrolytic dissociation of the solution. It will normally besufficient to maintain the current density at the solution-electrodeinterface below 3.5 f-5.5 A/cm², where f is the frequency of thealternating current in sec.⁻¹. At a supply system frequency of 50 Hz,the maximum current density is then about 0.45 A/cm².

It has been found that the evaporation rate can easily be controlled byan automatic control of the overflow level of the concentrated solutionbecause this results in a variation of the cross-section of the solutionand, if the voltage and the solution resistance are constant, in avariation of the electric current and electric power.

If an increase in density results in a lower overflow level, the densitymay be automatically controlled in a simple manner. In case of anexcessively high power, the density increases and with it decreases thecurrent-carrying cross-section of the solution flowing through theevaporator, so that the evaporation rate will also decrease. As aresult, the density is reduced and the automatic control action takesplace in the opposite sense. It will be understood that, alternatively,the evaporation rate may be controlled by a transformer, thyristor, orthe like.

A complete explanation of the solubility relations in the presentsolution system would be an enormously difficult task, not only becausesuch a large number of different ions are involved but also becuase ironfluoride can occur in the solid phase in a large number of differentforms, and a plurality of crystallites tend to remain in existencesimultaneously although only one crystal form is in thermodynamicequilibrium.

Nevertheless, a detailed consideration of these solutions revealscertain regular phenomena, which involve a kinetic activity that issignificant in practice. In this connection it is mainly significantthat a satisfactory yield of crystals decisively depends on the presenceof fluoride ions in an amount which is sufficient for combining with themetal ions also present. For this reason, another rule which isdesirably followed calls for a continuous or discontinuous addition ofhydrofluoric acid to the circulating solution at any desired point inorder to maintain the fluoride ion normality of the concentrate alwaysabove the sum of the metal ion normalities before crystallizationbegins.

It has now been found that hydrofluoric acid is predominantly distilledoff even in preference to a large surplus of nitric acid. For thisreason it is desirable in most cases to add the make-up hydrofluoricacid to the solution cycle after the distillation and beforecrystallization begins.

Different considerations are applicable to nitric acid. It is known thatalmost all nitrates are very highly soluble. In the present system, inwhich nitrates are associated with hydrofluoric acid, the nitrateconcentration has virtually no influence on the precipitation ofcrystals. Because some mother liquor is separated with the precipitationof crystals, the resulting loss of nitric acid is proportional to thenitric acid concentration in the concentrated solution which isavailable before the crystallization begins. For this reason it is notadvisable, as in the case of hydrofluoric acid, to add nitric acid atthis stage. According to a preferred feature of the invention, themake-up nitric acid is added to the circulating solution after thecrystals have been separated. To this end, the acid may be added to themother liquor or fed directly into the metal-treating bath.

The economy of the process depends also on the time in whichcrystallization is allowed to take place in the concentrated solution.It has been found that a time interval of 6 to 30 hours, preferably 8 to24 hours, is preferably allowed to elapse between the distillation andthe separation of crystals.

Apparatus which can be used to carry out the process which has beendescribed above comprises one or more continuous or batch evaporators,condensers, crystallizers, crystal separators, fittings and pipelinesfor connection to the metal-treating (pickling) plant. This equipmentconsists of or is lined with materials which are resistant to nitric andhydrofluoric acids. Examples of such materials arepolyfluorohydrocarbons, sintered corundum and structural graphite. Adesirable evaporator consists of one or more flow channels formed bysubstantially horizontally extending, straight or curved pipes, whichare circular or profiled in cross-section and which may have an upwardlyflaring, trapezoidal cross-section for receiving foam, vapor off-takesfor the evaporated liquid, inlets and outlets for the solution flowingthrough, and a plurality of electrodes, which are spaced apart along theflow channels and immersed in said solution and consist, e.g., ofgraphite and are provided with terminals and serve for a directresistance heating of the spent pickling solution with alternatingcurrent.

Because the electrodes assume a high temperature in operation, anyportion of an electrode which protrudes from said solution will promoteformation of undesirable crusts and may even cause undesirableintroduction of seed crystals. For this reason, a desirable embodimentcomprises electrodes which are introduced from the bottom or from thesides of the evaporator and are entirely submerged in the solution.

As has been mentioned, it will often be desirable from the aspect ofprocess technology to divide the distillate into a plurality offractions and to use a first of these fractions, e.g., as rinsing water,whereas a second and any subsequent fractions are returned to thecirculating solution. Such division involves no difficulties whenevaporation is carried out batch-wise. In the case of continuousevaporation, the space over the aforesaid flow channels of theevaporator must be divided for this purpose into a plurality ofcompartments, such as consecutive compartments. A continuous evaporatorsuitable for this purpose has one or more transverse internal partitionsabove the body of flowing solution which divide the gas space thereover,separate vapor off-takes being provided for the respective compartments.It will be understood that with separate condensation of the resultingvapor fractions the aforementioned wash water and recyclable picklingsolution make-up can be achieved.

Referring now to the nature of a desirable crystallizer, it must beremembered that the crystal habits sometimes tend to undesirably adhereto the walls of the crystallizer. For this reason it is preferred to usea simple crystallizer. However, since such a crystallizer has, bydefinition, walls which are closely spaced apart, the immersed surfacesthereof are best wiped off, continuously or periodically, to preventbuild-up of crystals thereon.

Typically, the crystallizer consists of an upright cylindrical vesselhaving an inlet and an outlet, a driven shaft which extends along theaxis of rotation of the vessel and is profiled by being provided withradial ribs extending along said axis. One or more circular columns(roll-like upright members) are provided, which stand on the bottom ofthe vessel and are positioned between the driven shaft and the innerwall of the vessel, which upright members are rotatable and providedwith ribs, arranged in a star-shaped configuration and extending alongthe axis of each of said members. These roll-like members are sopositioned as to contact the side wall of the vessel and mesh with theribs of the driven shaft so that each roll-like member rotates on itsown axis and revolves in the opposite sense in the annular space betweenthe drive shaft and the side wall of the vessel, whereby all immersedsurfaces are wiped so as to wipe off adhered crystals or to maintain thethickness of adhered crystals below 10 mm, preferably 5 mm.

Practical experiments have shown that the drive shaft of thecrystallizer is preferably profiled by the provision of 1 to 4 ribs andeach of the members rising from the bottom is preferably profiled by theprovision of 5 to 8 ribs.

Further details of the process and apparatus will now be explained morefully with reference to the drawings, in which

FIGS. 1 to 3 are diagrams which illustrate the crystallization rate.

FIG. 4 illustrates the relation between the highest permissible currentdensity at the electrodes and the frequency of the alternating current.

FIG. 5 is a flow sheet of block diagrams illustrating a preferredembodiment of the process of the present invention.

FIGS. 6 to 8 are sketches which represent illustrative embodiments ofthe apparatus.

Specifically,

FIGS. 6 and 7 are diagrammatic views showing, by way of example,different sectional views of a trough evaporator according to theinvention, which evaporator operates with a sharply-defined, short-timeresidence time spectrum for the solution to be evaporated, FIG. 6 beinga sectional view taken along line VI--VI in FIG. 7 and FIG. 7 asectional view taken along line VII--VII in FIG. 6, and

FIG. 8 is a horizontal sectional view illustrating the principal of acrystallizer according to the invention.

FIG. 1 shows the approximate relation between the iron concentration (gFe/l) resulting from evaporation and the time lapse (hrs) between theend of evaporation and start of crystallization.

It is apparent that the evaporation can be carried out to effect alarger or smaller increase in concentration, depending on the residencetime of the solution in the evaporator and on the sharpness or width ofthe residence time spectrum. The use of conventional evaporators hardlyenables an evaporation to a concentration of more than 80 g/l Fe or, atmost, 90 g/l Fe in the concentrated solution. High-intensity continuoustrough evaporators may be used for an evaporation to a concentration of,e.g., 110 g/l Fe if the solution flowing through is subjected to theevaporation process for only a few minutes. This indicates that the useof a properly designed evaporator is essential because only a high ironconcentration enables a fast crystallization with a high yield.

The influence of temperature and the change of temperature on the Fe⁺⁺⁺concentration in the solution during the crystallization is apparentfrom FIG. 2. A solution containing 80 g Fe⁺⁺⁺, 14 g Cr⁺⁺⁺, 6 g Ni⁺⁺⁺,about 280 g NO₃ ⁻ and 150 g F⁻ was divided into three portions, each ofwhich was treated while being stirred. As is indicated by a dash-dotline, the first portion was initially cooled to 20° C. and then left atthis temperature. The dotted line indicates that the second portion wascooled to 50° C. and held at this temperature. A solid line indicatesthat the third portion was initially cooled to 50° C. and then permittedto cool slowly to room temperature.

As this was only a basic test, the temperature-time function has notbeen recorded. In any case the tests show clearly the surprisingbehavior of supersaturated solutions of this kind when subjected toretarded crystallization. A fast cooling is undesirable from the aspectof crystallization technology. On the other hand, a slow cooling resultsinitially in a higher crystallization rate, which is due to the highertemperature, and permits subsequently of a utilization of the lowersolubility at a lower temperature.

For this reason, solutions actually available in practice can be used inthe laboratory for defining an optimal temperature-time function, whichcan be program-controlled. It is believed, however, that a slow coolingof, e.g., 100 liters of solution during several hours with exposure tothe air, is sufficiently close to the optimum so that the considerableexpenditure involved in a time program-controlled cooling is notrequired. A similar time function is exhibited, e.g., by theconcentration of dissolved chromium.

FIG. 3 shows the decrease of the Fe⁺⁺⁺ content of the same solutionduring the slow cooling to room temperature in the presence of differentcontents of free hydrofluoric acid. In the present medium, which has ahigh nitric acid content, the content of free hydrofluoric acidcorresponds to the normality of the fluoride ion concentration less thesum of the metal ion normalities. It is apparent that thecrystallization is strongly accelerated by a surplus of 60 to 70 g/lhydrofluoric acid. The addition of more hydrofluoric acid does notresult in a substantial further improvement and may sometimes evenproduce undesirable results, possibly by the formation of complexes.

The direct electric heating of electrolyte-containing solution mayresult in an electrolytic dissociation of the solution. For this reason,the upper limits of the current densities at platinum electrodes havebeen measured at different a.c. frequencies. These upper limits of thecurrent densities are the highest current densities which can beemployed for a prolonged time without an evolution of gas. It has beenfound that these current density limits, like the actual over-voltages,are related to the a.c. frequency by exponential functions, which areshown in FIG. 4 and which differ for the ranges above and below 5000 Hz,respectively.

Only the lower part of the curve is of practical significance and isfairly exactly in accordance with the above-mentioned formula andindicates a maximum permissible current density of 0.45 A/cm² at theelectrodes at a supply system frequency of 50 Hz. A slightly higherfrequency may be used with the usual graphite electrodes, depending onthe specific material. It is believed that in this case a certaininfluence is also exerted by the effective surface area, which exeedsthe geometric surface area. In any case, an operation in the safe rangeis possible with the above values determined for platinum electrodes.

The size of the electrode does not normally constitute a limiting designvalue, and the electrode material will not be significant for the costs.For this reason, the low current density which is permissible at asupply system frequency of 50 Hz will usually be considered sufficientbecause the costs of a generator for higher frequencies need not beincurred in that case.

In the process illustrated in the flow sheet of block diagrams of FIG.5, a high-grade steel strip 1 is pickled as it is pulled through anitric-hydrofluoric acid bath 2 and is then rinsed twice in two rinsingstages 3 and 4. The used solution is evaporated to a higherconcentration in two stages 5 and 6. Make up hydrofluoric acid 7 in therequired amount is added to the concentrated solution as it iscirculated. The concentrated solution is then allowed to cool slowly ina crystallizer 8. When a cake 10 consisting of moist FeF₃, CrF₃ etc. hasbeen separated, the mother liquor is recycled to the pickling vessel 2by a pump 11. Nitric acid 12 required to make up the circulatingsolution is added directly to the pickling bath 2.

The distillate withdrawn from the pre-evaporator 5 is indirectly cooledwith cooling water 13 in a heat exchanger 43. The resulting condensateis virtually pure water, which can be used in the second rinsing stage4, to which additional rinsing water 14 must be added in most cases. Therinsing water used in this stage is then used in the first rinsing stage3. The only slightly acidic distillate from the second evaporator 6 mayalso be used in the stage 3 when this distillate has been condensed bybeing indirectly cooled with water 15 in a heat exchange 45.Alternatively, it may be preferred to feed said distillate directly intothe pickling bath 2.

The effluent from the first rinsing stage 3 serves to fill up the volumeof the pickling bath 2. Any surplus effluent 16 not required for thispurpose must be subjected to sewage treatment.

The evaporator 17 shown in FIGS. 6 and 7 has approximately the shape ofa prism which tapers to form a trough 18 and in which the solution 19received at one end 20 is evaporated as it flows through said trough.The effluent is conducted through a flexible conduit 21. Four carbonelectrodes 22 are engaged by annular discs 23, which are guided by pins46 and are biased by coil springs 24 tending to hold the electrodes insealed engagement with the edges of circular apertures 47 in the bottom48 of the evaporator. The phase conductors R, S, T, R etc. of thethree-phase a.c. supply system used by way of example are connected tosuccessive ones of these electrodes. The electrodes 22 are contacted bythe solution only at the slightly convex end faces of the carbonelectrodes. A transverse partition 25 defines two separate vaporcompartments 26, 27, from which two respective distillates 28, 29 can beseparately withdrawn.

It is essential to provide wide off-take conduits and to ensure anequalization of pressure so that the surface of the solution beingdistilled is not subjected to differential pressure by the vapors. Anoverflow vessel 31 is suspended from a spring 30 and provided with aflexible pressure-equalizing conduit 32 and a drain conduit 33. Thisoverflow vessel rises and falls in dependence on the density orconcentration of the concentrated solution. As the concentration anddensity of the solution increase, the overflow vessel falls and with itthe level of the solution flowing in the evaporator so that thecross-section of the solution 19 is decreased. When the voltage isconstant and the resistivity of the solution changes only slightly, theresistance and with it the electric power and the rate at which liquidis evaporated is automatically controlled in such a manner that thedensity of the outflowing concentrated solution is maintained constantand the salt concentration of said solution is approximately constant.This simple automatic control system is merely illustrative, since manyothers can be employed.

The crystallizer shown in FIG. 8 comprises a barrel-shaped vessel 34which contains a central vertical driven shaft 35, which, in turn, isprovided with four wing-like radial ribs 38. The vessel is also providedwith a vertical circular column 36 which stands on the bottom of thevessel, is freely rotatable, and provided with eight radial ribs 37creating a star-shaped configuration. The column 36 is rotated about itsaxis by drive shaft 35 which likewise rotates about its own axis and isin rib-meshing (gear-like) relationship with said column. A plurality ofcolumns 36 may be provided. The number of ribs 38 on drive shaft 35 andthe number of ribs 37 on column 36 is optional. In a preferredarrangement, shaft 35 is provided with one to four ribs and column 36 isprovided with five to eight ribs. Since the ribs, which are in loosemesh relationship, contact the inner bottom and the inner side wall ofthe vessel, removal of adhering crystal deposits therefrom or preventionof build-up to a substantial thickness is assured. The detached crystalsgravitate to the bottom and can be discharged therefrom continuously orintermittently.

It is apparent from the above description that for the first time simpleand economical steps are involved with maximum recovery of valuablechemicals. For example, the process of the present invention does notinvolve the formation of a heavy metal salt solution, which must bereprocessed, or of a hydroxide mud, which can be filtered only withdifficulty, but involves the formation of a compact cake of precipitatedfluorides, which in that form can be sent directly to a processor or maybe processed with simple equipment in the system from which the cake wasderived.

Typical of the simplicity and economy, also, are the fact that, ashereinabove suggested: at least the first distillation cut can be usedto rinse the metal stock; used rinsing water can be added to the nitricacid treating solution; moist crystals which have been separated bye.g., filtration can be extracted with a small amount of water or usedrinsing water, and nickel and, preferably, chromium and molybdenum aswell, can be recovered from the resulting solution by known and facilemethods, such as by selective precipitation, ion exchange or organicsolvent extraction.

The selective precipitation and separation of nickel hydroxide and itsprocessing for use in the production of high-grade steel is particularlyrecommended because nickel is very expensive. It will be understood thatthe crystals can be rendered substantially insoluble by being mixed withlime, whereby even hydrofluoric acid is transformed into insolublecalcium fluoride, and the insoluble material can then be dumped.

The crystals which have been extracted with water are sufficiently freefrom nitric acid and may be transformed into metal sulfates and hydrogenfluoride gas by a treatment with sulfuric acid at elevated temperatures.The hydrogen fluoride gas can be returned to the circulating solution.In most cases, however, it will be more desirable to adopt a modifiedprocedure in which the crystals are heated to 200° to 600° C.,preferably 300° to 400° C., whereby the presence of water in the form ofmoisture and water of crystallization cause the fluorides to behydrolytically decomposed into solid metal oxides and gaseous hydrogenfluoride. The hydrogen fluoride is absorbed in water or in an aqueoussolution - in many cases it is sufficient to use the water which iscondensed by the indirect cooling from distilled-off vapors - and thethus absorbed hydrogen fluoride can be returned to the circulatingsolution.

Alternatively, the crystals formed may be prohydrolyzed, in the presenceof aqueous vapour, in a furnace heated to 500° C. Iron fluoride isdecomposed into iron oxide and hydrofluoric acid.

Other similar economies and efficiencies are inherent and will beobvious to the person skilled in the art.

EXAMPLES

The present invention will best be understood from the followingexamples which are not meant to unduly limit the scope of the inventionbut which are intended to be illustrative thereof:

EXAMPLE I

During the pickling of 10 tons of high-grade steel strip, 10 kg Fe, 2Cr, and 1 kg Ni are dissolved in 397 kg of an aqueous solution of 3.9 kgFe, 1.1 Kg Cr, and 6.7 kg Ni in 66.2 kg HNO₃ and 24.8 kg HF. This isaccompanied by a decomposition or evaporation of 20 kg HNO₃ and by aremoval of 0.2 kg HF and 60 kg H₂ O with the vapors sucked from thebath. The used rinsing water contains 0.9 kg Fe, 0.2 kg Cr, and 0.5 kgNi dissolved in 3 kg HNO₃, 1.6 kg HF and 18 kg water. As a result, 345.6kg of solution are subjected to distillation; this solution contains 13kg Fe, 2.9 kg Cr and 7.2 kg Ni dissolved in 43.2 kg HNO₃ and 23 kg HF.1.4 kg HNO₃, 2.2 kg HF and 142 kg water are distilled off. When thecrystallization has been terminated, eight hours after the evaporation,43.2 kg of a filter cake are separated, which contains 9.1 kg Fe, 1.8 kgCr, and 0.5 kg Ni, which are mainly combined with 3 kg HNO₃ and 11.4 kgHF. The distillate is added to the filtrate, also a solution of 26 kgHNO₃ and 13.2 HF (calculated as 100% acid) in 55 kg water, so that aregenerate is obtained which amounts to 396.6 kg and consists of 3.9 kgFe, 1.1 kg Cr and 6.7 kg Ni dissolved in 66.2 kg HNO₃ and 24.8 kg HF,balance water. Because the nickel fluoride is more soluble, it becomesenriched in the circulating solution until an equilibrium has beenattained, as in this example. This solution has nevertheless anexcellent pickling activity. All quantities stated in kilograms relateto one hour of operation. The system is in dynamic equilibrium. The acidquantities relate to sums, i.e., nitric acid and nitrates are referredto as "nitric acid"; the same applies the hydrofluoric acid.

EXAMPLE II

Into an evaporator having a heating capacity of 135 kWh/hr is fed aspent pickling solution at the rate of 78 gals/hr, i.e., 780 lbs/hr, andhaving the following profile:

    ______________________________________                                        wt %                      lbs/hr                                              ______________________________________                                        3.4          Fe           26.5                                                1.1          Cr           8.6                                                 1.6          Ni           12.5                                                12.0         NO.sub.3     93.6                                                6.0          F'           46.8                                                75.9         H.sub.2 O    592.0                                               ______________________________________                                    

The spent pickling solution from the pickling plant has a temperature ofabout 50° C. In the evaporator it is rapidly heated to boiling, i.e.,105° C., and, as concentration increases, the temperature rises to about107.5° C.

The concentrate emerges from the evaporator at a rate of 34.8 gals/hr(407 lbs/hr) and has the following profile:

    ______________________________________                                        wt %                      lbs./hr                                             ______________________________________                                        6.5          Fe           26.5                                                2.1          Cr           8.6                                                 3.1          Ni           12.5                                                22.1         NO.sub.3     89.9                                                10.1         F'           41.2                                                56.1         H.sub.2 O    228.3                                               ______________________________________                                    

The vapors, on the other hand, are recovered as distillate by continuouscondensation in a heat exchange (condenser) using cool water.

The vapors emitted from the vaporizer are introduced into a condenser(using 30° F. H₂ O) and collected as distillate at a rate of 44.7gals/hr (373 lbs/hr). The distillate has the following profile:

    ______________________________________                                        wt %                      lbs/hr                                              ______________________________________                                        1            NO.sub.3     3.7                                                 1.5          F'           5.6                                                 97.5         H.sub.2 O    363.7                                               ______________________________________                                    

As shown in FIG. 5 of the drawing and discussed heretofore, thecondensate is recycled to the pickling zone.

The supersaturated concentrate from the evaporator, prior to being fedto crystallizers, is cooled to a temperature of about 75° C.

Two crystallizers are used and on introduction into the crystallizersthe temperature is again reduced to about 50° C. In the crystallizersthe concentrate is stirred constantly for about 12 hours at about 50° C.temperature; then it is cooled to about 20° C., and while still stirringit is kept in the crystallizers for another 12 hours before effectingseparation of the crystals formed from their mother liquor.

A total of 80 lbs/hr filter cake is removed by filtration, the cake'sprofile being:

    ______________________________________                                        wt %                      lbs/hr                                              ______________________________________                                        25.0         Fe           20.0                                                4.6          Cr           3.7                                                 0.8          Ni           0.6                                                 6.0          NO.sub.3     4.8                                                 30.8         F'           24.6                                                32.9         H.sub.2 O    26.3                                                ______________________________________                                    

The filter cake is neutralized with lime which is admixed therewith atthe rate of 38.4 lbs/hr. The neutralized product collected at the rateof 90.4 lbs/hr has this profile:

    ______________________________________                                        wt %                      lbs/hr                                              ______________________________________                                        31.6         Fe.sub.2 O.sub.3                                                                           28.6                                                6.0          Cr.sub.2 O.sub.3                                                                           5.4                                                 0.9          NiO          0.8                                                 7.0          Ca(NO.sub.3).sub.2                                                                         6.3                                                 54.5         CaF.sub.2    49.3                                                ______________________________________                                    

The mother liquor (filtrate) is separated by filtration at the rate of32.7 gals/hr (327 lbs/hr) has this profile:

    ______________________________________                                        wt %                      lbs/hr                                              ______________________________________                                        2.0          Fe           6.5                                                 1.5          Cr           4.9                                                 3.5          Ni           11.9                                                26.0         NO.sub.3     85.1                                                5.1          F'           16.6                                                61.7         H.sub.2 O    202.0                                               ______________________________________                                    

This filtrate is combined with the distillate aforementioned from theheat exchanger (condenser) and, at the rate of 73 gals/hr (700 lbs/hr)fed to the pickling zone. The combined filtrate/distillate condensateprofile follows:

    ______________________________________                                        wt %                      lbs/hr                                              ______________________________________                                        0.9          Fe           6.5                                                 0.7          Cr           4.9                                                 1.7          Ni           11.9                                                12.7         NO.sub.3     88.8                                                3.2          F'           22.2                                                80.8         H.sub.2 O    565.7                                               ______________________________________                                    

Of course, additional HNO₃ /HF/H₂ O, at the following rates are added tothe filtrate/distillate to form the pickling solution:

    ______________________________________                                         lbs/hr                                                                       ______________________________________                                        43                   HNO.sub.3                                                32.2                 HF                                                       261.1                H.sub.2 O                                                ______________________________________                                    

Pursuant to statutory requirements, there are described above theinvention and what are now considered its best embodiments. It should beunderstood, however, that the invention can be practiced otherwise thanas specifically described, within the scope of the appended claims.

What is claimed is:
 1. In the process of pickling metals with mixedaqueous solutions containing hydrofluoric acid and nitric acid bycontacting the metals with the solutions in a pickling zone andrecovering components of resultant spent pickling solutions, said spentsolutions containing metal dissolved during pickling, including iron,the improvement which comprises:(i) rapidly evaporating the spentsolution at boiling temperature in an evaporation zone to form asupersaturated concentrate containing 60 to 150 g/l Fe; (ii) within lessthan 60 minutes from the commencement of evaporation and upon achievingthe 60 to 150 g/l Fe desideratum, cooling the concentrate a first timeto a temperature in the range of about 50° C. to about 100° C. andtransferring it to a crystallization zone; then cooling the concentratea second time, in the crystallization zone, to a temperature in therange of about 30° C. to about 70° C., and maintaining, while stirring,this temperature for a period of 3 to 15 hours to effectcrystallization; (iv) lowering the temperature of the concentrate athird time, in the crystallization zone, below the cooling temperatureof the second cooling and to a range of about 15° C. to about 30° C. andmaintaining, while stirring, this temperature for a period of 3 to 15hours to effect further crystallization; and (v) separating theresulting crystals from resultant concentrate mother liquor, whichmother liquor is suitable for additional pickling purposes.
 2. Theprocess of claim 1 in which, if necessary, additional aqueoushydrofluoric acid is added to the supersaturated concentrate beforecrystallization begins, the amount added being sufficient to maintainthe fluoride ion normality of the concentrate above the sum of the metalion normalities.
 3. The process of claim 1 wherein the separation instep (v) is carried out by filtration.
 4. The process of claim 1 inwhich evaporation of the spent pickling solution is achieved by heatsupplied by direct electrode electrical resistance heating withalternating current, the current density at the solution-heatingelectrode interface being not in excess of 3.5 log f - 5.5 amperes persquare centimeter where f is the frequency of the alternating current insec.⁻¹.
 5. The process of claim 1 wherein hydrofluoric acid is added toany desired point to maintain the fluoride ion normality of the spentpickling solution always in excess of the sum of the metal ionnormalities.
 6. The process of claim 5 wherein the excess is about 10 to50 g/l hydrofluoric acid.
 7. The process of claim 1 wherein make-upnitric acid for compensating consumption of nitric acid is added to themother liquor after step (v).
 8. The process of claim 1 wherein themoist crystals which have been separated are washed with a small amountof water and at least nickel is recovered from the washing water, thecrystals washed with water are heated to 200° to 600° C. to releasehydrogen fluoride gas which is absorbed in an aqueous solution, saidaqueous solution then being, optionally, used to provide circulatingadditional hydrofluoric acid solution.
 9. The process of claim 1 whereinan essentially aqueous distillate, a first cut from the evaporationzone, is used as rinse water in the pickling zone.
 10. The process ofclaim 1 wherein acidic distillate from the evaporation zone is used inthe pickling zone.
 11. The process of claim 1 wherein thecrystallization zone is made of two or more successive zones and theconcentrate flowing therethrough is maintained at the indicatedtemperatures in the first zone or zones and at the lower temperature inthe subsequent zone or zones, the concentrate flow through these zonesbeing at a rate which provides uniform residence times in the separatetemperature zones.
 12. The process of claim 1 wherein spent picklingsolution is made to flow through an evaporating zone which iscompartmentalized to collect distillate fractions having differentcomponent profiles.
 13. The process according to claim 1 in which theevaporation rate in step (i) is controlled to maintain a constantoverflow level of the concentrated solution.
 14. In the process ofpickling metals with mixed aqueous acid solutions containinghydrofluoric acid and nitric acid by contacting the metals with thesolutions and recovering components of resultant spent picklingsolutions, said spent solutions containing metal dissolved duringpickling, including iron, the improvement which comprises:(i) rapidlyevaporating the spent solution at boiling temperature to form asupersaturated concentrate containing 80 to 120 g/l Fe; (ii) within lessthan 15 minutes from the commencement of evaporation and upon achievingthe 80 to 120 g/l Fe desideratum, cooling the concentrate a first timeto a temperature in the range of about 70° C. to about 78° C. andtransferring it to a crystallization zone; (iii) then cooling theconcentrate a second time, in the crystallization zone, to a temperaturein the range of about 40° C. to about 60° C., and maintaining, whilestirring, this temperature for a period of 4 to 12 hours to effectcrystallization; (iv) lowering the temperature of the concentrate athird time, in the crystallization zone, below the temperature of thesecond cooling and to a range of about 18° C. to about 25° C. andmaintaining, while stirring, this temperature for a period of 4 to 12hours to effect further crystallization; and (v) separating theresulting crystals from resultant concentrate mother liquor, whichmother liquor is suitable for additional pickling purposes.